After getting some requests for the F5 cascoded version I made a while ago, I ordered a small batch. You can read about toxic ingestion's successful build here:
http://www.diyaudio.com/forums/pass-labs/260750-f5c-cascode-cviller-boards-40-v-rails-build.html
Spoiler alert: He experiences oscillations, which is fixed by adding a capacitor between gate and output. I have now added a place for those capacitors to the board.
The issue and a lot more is described in papa's excellent article about hot rodding the F5: http://www.firstwatt.com/pdf/art_f5_turbo.pdf thread (must read article).
To avoid dealing with tons of mails, I have set up a resurrected my old site where you can order the boards:
cviller order site
(please let me know if you run into any issues).
http://www.diyaudio.com/forums/pass-labs/260750-f5c-cascode-cviller-boards-40-v-rails-build.html
Spoiler alert: He experiences oscillations, which is fixed by adding a capacitor between gate and output. I have now added a place for those capacitors to the board.
The issue and a lot more is described in papa's excellent article about hot rodding the F5: http://www.firstwatt.com/pdf/art_f5_turbo.pdf thread (must read article).
To avoid dealing with tons of mails, I have set up a resurrected my old site where you can order the boards:
cviller order site
(please let me know if you run into any issues).
I've F5 boards of version V2.0 and I use 2x24Vac toroid which gives around 34Vdc rail voltage.I want to use cascode could someone explain me the cascode calculation.You can mail me 1964Roovers@gmail.com
I would really recommend reading the F5 turbo article. It has some good info about cascode calculation as well.
http://www.firstwatt.com/pdf/art_f5_turbo.pdf
http://www.firstwatt.com/pdf/art_f5_turbo.pdf
The MOSFETs need about 5V to be comfortably biased where you need them.
The JFETs are happiest with 10-12V (enter low noise region below 12V Vds) across them.
To have a bit of wiggle room assume the cascode needs to take care of the total supply voltage minus the sum of the above two, so you're looking at 16-18V, which is about half the supply voltage in your case.
I usually divide this by 1000 to get the total divider resistance, and then divide down to get the voltage ratio. You would use equal resistors on both sides to halve the voltage. One Vbe is to be added but I usually ignore it. The answer in your case would be 8k2 or 9k1 resistors. Any value between 5 and 15k would be fine, just use what you have in the junk box.
I've used lower and higher values without issues as the collector current of the cascode transistor is very small so base current typically will not exceed few uA. The thing to watch is collector dissipation and temperature rise, so ensure the thermal limits are well-respected.
That said, I have used puny TO-92 transistors here with no ill-effects at all at those voltages. Wouldn't trust them beyond 35V rails though, or with higher Idss JFETs.
The JFETs are happiest with 10-12V (enter low noise region below 12V Vds) across them.
To have a bit of wiggle room assume the cascode needs to take care of the total supply voltage minus the sum of the above two, so you're looking at 16-18V, which is about half the supply voltage in your case.
I usually divide this by 1000 to get the total divider resistance, and then divide down to get the voltage ratio. You would use equal resistors on both sides to halve the voltage. One Vbe is to be added but I usually ignore it. The answer in your case would be 8k2 or 9k1 resistors. Any value between 5 and 15k would be fine, just use what you have in the junk box.
I've used lower and higher values without issues as the collector current of the cascode transistor is very small so base current typically will not exceed few uA. The thing to watch is collector dissipation and temperature rise, so ensure the thermal limits are well-respected.
That said, I have used puny TO-92 transistors here with no ill-effects at all at those voltages. Wouldn't trust them beyond 35V rails though, or with higher Idss JFETs.
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